A human Hanganutziu-Deicher (H-D) serum reacted with N-glycolylneuraminic acid (NeuGc)-containing gangliosides and glycoproteins isolated from bovine erythrocyte membranes. Three populations of H-D antibodies were identified in the human H-D serum. One population very likely recognized the NeuGc-Gal sequence; a second population appears to recognize additional sugars in the oligosaccharide sequence, e.g. NeuGc-Gal-GlcNAc; while a third population may also recognize polypeptide determinants in addition to the NeuGc-Gal-GlcNAc sequence. The H-D serum distinguished two high mol. wt glycoproteins (HMGP I and II) present in crude extracts of bovine erythrocyte membranes. These glycoproteins were separated by repetitive fractionation on Sephacryl S-1000 in the presence of urea and their composition determined.